The solution might be more simple than we think....
Jack, Kate, Sawyer, Hurley, and the remaining folks on the island preoccupy and trick the smoke monster into sinking the island while he is still on it thereby trapping him forever. Maybe they push him back into the golden cave? :idunno: Or maybe the smoke monster sinks the island but our losties trap him in some way so that he sinks along with the island.
The downside is they all die in the process. Or do they?
As Jack, Kate, Sawyer, Hurley, and the gang deal with the smoke monster, Desmond must rush to "wake" everyone up in the ALT. By waking up our losties in the ALT it truly saves them by transporting them into a "safe" reality where their lives are not being threatened by the smoke monster. Yet the beauty is... those that "wake up" in the ALT will be our REAL losties from the island, they will have all their memories in tact. It will really be them, our favorite characters that we have grown to love over the last 6 years, not a ALT version that we cannot identify with. For instance, Hurley in the ALT is now awake thanks to Desmond and Libby, therefore Hurley is safe! He has escaped the island in tact! All Hurley needs to do now is let go. Let go of his island conciseness and when he does he is truly free and has escaped the island. Jin, Sun, Kate, Sawyer, Sayid, and even Jack and Locke can escape if they can "wake up" in the ALT and then let go of their island reality.
Therefore... Desmond needs to work quick to wake up the losties while our on-island hero's trick the smoke monster into trapping himself under water.